1. Field of the Invention
The present invention relates to an injection nozzle for feeding an injectable material to a mold cavity and, more particularly, to an injection nozzle which is capable of injecting difficult materials, such as polyethylene terephthalate (PET) plastic. Even more particularly, the present invention is directed to an injection nozzle having a guide sleeve for supporting and guiding a nozzle pin between a retracted position in which the exit orifice from the nozzle is open and an extended position in which the exit orifice is closed. The present invention further relates to an injection molding installation having such a nozzle.
2. Prior Art
One method for charging an injection nozzle with a moldable material is disclosed in U.S. Pat. No. 4,213,751, wherein a thermoplastic material is molded to form blanks which are subsequently formed into plastic bottles by a blow-molding process. An injection nozzle for use in this process is shown schematically in FIG. 1. In this arrangement, a nozzle 1 includes an axially extending nozzle channel section 7 which communicates with a nozzle orifice 5 arranged in nozzle head 3 for the delivery of a moldable material. A nozzle pin 9, coaxially arranged in nozzle channel 7, may be axially displaced in the direction of arrow A into or out from orifice 5. As used herein, a nozzle pin channel refers to the portion of the nozzle channel through which the nozzle pin travels in direct communication with the moldable material. Thus, referring to FIG. 1, the axial length of nozzle pin channel 6 is equal to the axial length of the nozzle channel 7 and is substantially longer than the diameter of nozzle pin 9.
The moldable material is injected radially in a hot, liquid state from a feed pipe 13 into nozzle channel 7 through an inlet opening 11 located at a spaced distance from orifice 5. Considering a cross-sectional area of nozzle channel 7 along line Q--Q, when the moldable material is injected through inlet opening 11 under pressure, a distribution of pressure p results around the circumference of nozzle pin 9, all of which is shown qualitatively in FIG. 1(b). The material flowing into injection nozzle 1 creates a pressure in nozzle channel 7 which is greater on the side of nozzle pin 9 facing inlet opening 11 than on the side of nozzle pin 9 facing away from inlet opening 11. As a result of this non-uniform pressure distribution, a bending stress is exerted on nozzle pin 9 which causes the nozzle pin to deflect from its original straight shape, depending on the position of nozzle pin 9 relative to the nozzle orifice 5. In particular, with the nozzle pin 9 retracted from orifice 5, the injection of moldable material under pressure deflects the free end or tip of nozzle pin 9, as shown in phantom lines in FIG. 1(a), so that it is no longer aligned with nozzle orifice 5. When nozzle pin 9 is then extended to seal nozzle orifice 5, this deflection causes both the tip of the nozzle pin and the nozzle orifice to wear excessively. The exertion of even a very small bending force in the retracted position of nozzle pin 9 will cause an intolerable deflection of the nozzle pin tip relative to its tight tolerances with nozzle orifice 5.
It is known from U.S. Pat. No. 4,412,807 that, in order to solve this problem, the nozzle pin may be guided in a sleeve until it is directly in front of the nozzle orifice. The nozzle channel runs along one side of the guide sleeve in a circularly shaped passage eccentrically disposed relative to the central axis of the nozzle, turning toward the nozzle orifice at the end of the guide sleeve. This arrangement has the disadvantage that the asymmetrical flow of the moldable material to the nozzle orifice causes asymmetrical friction and thus uneven heating of the conveyed material, thereby leading to asymmetries in the delivery of the material into a mold. This problem is exaggerated when processing PET materials, which are very sensitive to temperature changes and gradients.
It has further become known from the German laid-open specification OS 26 14 911 that in an injection nozzle having the same basic structure as that in U.S. Pat. No. 4,213,751, a sleeve may be provided up to the vicinity at which the moldable material enters the nozzle channel from the feed pipe Such structure, however, does not overcome the problems associated with an asymmetrical material feed since such pressure asymmetries extend further down the nozzle channel toward the nozzle orifice.
French application No. 2,333,633 discloses a method for charging an injection nozzle in which a nozzle pin is axially displaceable to open or close a nozzle orifice. The nozzle pin runs in a sleeve having a heating coil for heating the nozzle pin. An outer sleeve holds the heating coil and extends into the immediate vicinity of the nozzle orifice at which point the nozzle pin protrudes therefrom. The injection nozzle further includes a coaxial channel arranged around the sleeve and symmetrical to the nozzle pin with respect to the nozzle orifice axis. The moldable material is fed to the coaxial channel through pairs of passages arranged axially and symmetrically at a spaced distance from the orifice. This symmetrical feed prevents the formation of any pressure asymmetries, and the sleeve thus provided serves exclusively to protect the heating coil. As a result of this symmetrical feed, a bending stress does not develop.
The injection nozzles described above have limited application in that they are only suitable for injecting materials which have favorable flow characteristics over a wide temperature range. However, many materials are heat sensitive, such that overexposure to heat will destroy their physical properties. One such material which is affected deleteriously by heat is PET thermoplastic. PET is extremely temperature sensitive, has a very low coefficient of thermal conductivity, is pressure sensitive and changes its optical characteristics if turbulence occurs during processing. In view of these properties, PET is intolerant of other than very small temperature gradients and requires a relatively large and unobstructed flow path and is therefore nearly impossible to mold using conventional injection nozzles
There therefore exists a need for a simple, high-quality injection nozzle which will not have a deleterious effect on the material being injected, which will provide an injected workpiece which is defect free, and which will overcome the many deficiencies previously encountered in prior art injection nozzles.